Biaxially oriented polyester film having a high oxygen barrier and process for its production

ABSTRACT

A metalized or ceramic-coated biaxially oriented polyester film is disclosed, with a base layer B which comprises at least 80% by weight of thermoplastic polyester, and with at least one outer layer A. The outer layer A is composed of a copolymer or of a mixture of polymers/copolymers which contains from 90 to 98% by weight of ethylene 2,6-naphthalate units and up to 10% by weight of ethylene terephthalate units, and/or units derived from cycloaliphatic or aromatic diols and/or dicarboxylic acids. The thickness of the outer layer A is more than 0.7 μm and this makes up less than 25% by weight relative to the entire film. The T g 2 value of the polyester film is above the T g 2 value of the base layer, but below the T g 2 value of the outer layer. The film has low permeability to atmospheric oxygen and exhibits very good adhesion between the individual layers. It is particularly suitable for packaging purposes, specifically for packaging foods or other consumable items.

BACKGROUND OF THE INVENTION

[0001] The invention relates to a transparent, biaxially orientedpolyester film with a base layer B which comprises at least 80% byweight of thermoplastic polyester, and with at least one outer layer Aand with a metallic or ceramic layer arranged on the outer layer A. Theinvention further relates to the use of the film and to a process forits production.

Prior art

[0002] EP-A-0 878 298 describes a biaxially oriented polyester film witha base layer B, at least 80% by weight of which is composed of athermoplastic polyester, and with an outer layer A and with a metallicor ceramic layer arranged on the outer layer A. The outer layer A ofthis film is composed of a mixture of polymers which contains at least60% by weight of ethylene 2,6-naphthalate units (PEN), up to 40% byweight of ethylene terephthalate units (PET) and optionally up to 10% ofunits derived from cycloaliphatic or aromatic diols and/or dicarboxylicacids.

[0003] If the metallized or ceramic-coated outer layer A of the film ofEP-A-0 878 298 contains high concentrations of ethylene 2,6-naphthalateunits, the film has a tendency for delamination between the outer layerA and the base layer B. If, on the other hand, the outer layer Acontains low concentrations of ethylene 2,6-naphthalate units, thethickness of this layer has to be raised in order to achieve the desiredlow oxygen permeation of not more than 0.3 cm³/(m²·bar·d).

[0004] In a film in Example 8 of EP-A-0 878 298 the metallized outerlayer A uses pure polyethylene 2,6-naphthalate (corresponding to 100% byweight of ethylene 2,6-naphthalate units). In this case there is nosignificant adhesion between the outer layer A and the base layer B. Thefilm is unsuitable for industrial use (e.g. as a composite film), sincethe bond releases even when subjected to a low level of mechanicalstress, due to the low adhesion between the outer layer A and the baselayer B of the polyester film.

[0005] In a film in Example 11 of EP-A-0 878 298, the metallized outerlayer A contains 60% by weight of ethylene 2,6-naphthalate units. Inorder to achieve the low oxygen permeation demanded, below 0.3cm³/(m²·bar·d), the thickness of the outer layer A has to be raised to2.5 μm, and this is economically disadvantageous (high capitalexpenditure and high material costs).

[0006] U.S. Pat. No. 5,795,528 describes a coextruded film laminatewhich has alternating layers of PEN and PET. Like the film of EP-A-0 878298, this film has a tendency toward delamination between the individuallayers of PEN and PET. There is no significant adhesion between theselayers. A laminate of this type is therefore again unsuitable forindustrial use.

[0007] It was an object of the present invention, therefore, to providea metallized or ceramic-coated, biaxially oriented polyester film whichovercomes the disadvantage of the prior art films and in particular hasimproved adhesion between the individual layers. It should be simple andcost-effective to produce, have very good barrier properties, and poseno problems of disposal.

SUMMARY OF THE INVENTION

[0008] The object is achieved by means of a biaxially oriented polyesterfilm with a base layer B which comprises at least 80% by weight ofthermoplastic polyester, with at least one outer layer A and with ametallic or ceramic layer arranged on the outer layer A, thecharacterizing features of which are regarded as being that

[0009] the outer layer A is composed of a copolymer or of a mixture ofpolymers/copolymers, which contains from 90 to 98% by weight of ethylene2,6-naphthalate units and up to 10% by weight of ethylene terephthalateunits, and/or units derived from cycloaliphatic or aromatic diols and/ordicarboxylic acids;

[0010] the thickness of the outer layer A is more than 0.7 μm and thismakes up less than 25% by weight of the entire film, and the T_(g)2value of the polyester film is above the T_(g)2 value of the base layerbut below the T_(g)2 value of the outer layer.

[0011] The metallized or ceramic-coated film of the invention has anoxygen permeation below 0.3 cm³/(m²·bar·d), and minimum adhesion betweenthe individual layers of the film of greater than or equal to 0.5 N/25mm.

DETAILED DESCRIPTION OF THE INVENTION

[0012] The structure of the film of the invention has at least twolayers, and is then composed of the metallized or ceramic-coated outerlayer A, and of a base layer B.

[0013] Preference is given to a polyester film in which the copolymer orthe mixture of polymers of the outer layer A contain from 91% to 97% byweight of ethylene 2,6-naphthalate units and up to 9% by weight ofethylene terephthalate units and/or units derived from cycloaliphatic oraromatic diols and/or dicarboxylic acids. Among these, particularpreference is in turn given to a polyester film in which the copolymeror mixture of polymers of the outer layer A contain from 92 to 96% byweight of ethylene 2,6-naphthalate units and up to 8% by weight ofethylene terephthalate units and/or units derived from cycloaliphatic oraromatic diols and/or dicarboxylic acids.

[0014] Preference is also given to a polyester film whose metallized orceramic-coated outer layer A has a thickness of more than 0.8 μm, wherethis makes up less than 22% by weight of the entire film, and particularpreference is given to a polyester film whose metallized orceramic-coated outer layer A has a thickness of more than 0.9 μm, wherethis makes up less than 20% by weight of the entire film.

[0015] Examples of suitable aliphatic diols are diethylene glycol,triethylene glycol, aliphatic glycols of the formula HO—(CH₂)_(n)—OH,where n is an integer from 3 to 6 (in particular 1,3-propanediol,1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol), or branchedaliphatic glycols having up to 6 carbon atoms, and cycloaliphatic diolshaving one or more rings and if desired containing heteroatoms. Amongthe cycloaliphatic diols, mention may be made of cyclohexanediols (inparticular 1,4-cyclohexanediol). Examples of other suitable aromaticdiols are those of the formula HO—C₆H₄—X—C₆H₄—OH where X is —CH₂—,—C(CH₃)₂—, —C(CF₃)₂—, —O—, —S— or —SO₂—. Besides these, bisphenols ofthe formula HO—C₆H₄—C₆H₄—OH are also very suitable.

[0016] Preferred aromatic dicarboxylic acids are benzenedicarboxylicacids, naphthalenedicarboxylic acids (for example naphthalene-1,4- or-1,6-dicarboxylic acid), biphenyl-x,x′-dicarboxylic acids (in particularbiphenyl-4,4′-dicarboxylic acid), diphenylacetylene-x,x′-dicarboxylicacids (in particular diphenylacetylene-4,4′-dicarboxylic acid) orstilbene-x,x′-dicarboxylic acids. Among the cycloaliphatic dicarboxylicacids, mention may be made of cyclohexanedicarboxylic acids (inparticular cyclohexane-1,4-dicarboxylic acid). Among the aliphaticdicarboxylic acids, the C₃-C₁₉-alkanedioic acids are particularlysuitable, where the alkane moiety may be straight-chain or branched.

[0017] The base layer of the film is preferably composed of at least 90%by weight of the thermoplastic polyester. Polyesters suitable for thisare those made from ethylene glycol and terephthalic acid (=polyethyleneterephthalate, PET), from ethylene glycol andnaphthalene-2,6-dicarboxylic acid (=polyethylene 2,6-naphthalate, PEN),from 1,4-bishydroxymethylcyclohexane and terephthalic acid(=poly-1,4-cyclohexanedimethylene terephthalate, PCDT), and also fromethylene glycol, naphthalene-2,6-dicarboxylic acid andbiphenyl-4,4′-dicarboxylic acid (=polyethylene 2,6-naphthalatebibenzoate, PENBB). Particular preference is given to polyesters whichare composed of at least 90 mol %, preferably at least 95 mol %, ofethylene glycol units and terephthalic acid units or of ethylene glycolunits and naphthalene-2,6-dicarboxylic acid units. The remaining monomerunits are derived from other diols and/or dicarboxylic acids. Examplesof suitable diol comonomers are diethylene glycol, triethylene glycol,aliphatic glycols of the formula HO—(CH₂)_(n)—OH, where n is an integerfrom 3 to 6, branched aliphatic glycols having up to 6 carbon atoms,aromatic diols of the formula HO—C₆H₄—X—C₆H₄—OH where X is —CH₂—,—C(CH₃)₂—, —C(CF₃)₂—, —O—, —S— or —SO₂—, or bisphenols of the formulaHO—C₆H₄—C₆H₄—OH are employed.

[0018] The dicarboxylic acid comonomer units are preferably derived from30 benzenedicarboxylic acids, naphthalenedicarboxylic acids,biphenyl-x,x′-dicarboxylic acids (in particularbiphenyl-4,4′-dicarboxylic acid), cyclohexane-dicarboxylic acids (inparticular cyclohexane-1,4-dicarboxylic acid),diphenylacetylene-x,x′-dicarboxylic acids (in particulardiphenylacetylene-4,4′-dicarboxylic acid), stilbene-x,x′-dicarboxylicacid or C₁-C₁₆-alkanedicarboxylic acids, where the alkane moiety may bestraight-chain or branched.

[0019] The polyesters may be prepared by the transesterificationprocess. The starting materials for this are dicarboxylic esters anddiols, which are reacted using the customary transesterificationcatalysts, such as salts of zinc, of calcium, of lithium and ofmanganese. The intermediates are then polycondensed in the presence ofwidely used polycondensation catalysts, such as antimony trioxide ortitanium salts. The preparation may be carried out just as successfullyby the direct esterification process in the presence of polycondensationcatalysts, starting directly from the dicarboxylic acids and the diols.

[0020] The copolymers for the outer layer A may be prepared in threedifferent ways:

[0021] a) In copolycondensation, terephthalic acid andnaphthalene-2,6-dicarboxylic acid are placed in a reactor together withethylene glycol, and polycondensed to give a polyester, using thecustomary catalysts and stabilizers. The terephthalate and naphthalateunits are then randomly distributed in the polyester.

[0022] b) Polyethylene terephthalate (PET) and polyethylene2,6-naphthalate (PEN), in the desired ratio, are melted together andmixed, either in a reactor or preferably in a melt kneader (twin-screwkneader) or in an extruder. Immediately after the melting,transesterification reactions between the polyesters begin. Initially,block copolymers are obtained, but as reaction time increases—dependingon the temperature and mixing action of the agitator—the blocks becomesmaller, and long reaction times give a random copolymer. However, it isnot necessary and also not always advantageous to wait until a randomdistribution has been achieved, since the desired properties are alsoobtained with a block copolymer. The resultant copolymer is thenextruded from a die and granulated.

[0023] c) Polyethylene terephthalate (PET) and polyethylene2,6-naphthalate (PEN) are mixed as granules in the desired ratio, andthe mixture is fed to the extruder for the outer layer A. Here, thetransesterification to give the copolymer takes place directly duringthe production of the film. This process has the advantage of being verycost-effective, and generally gives block copolymers, the block lengthbeing dependent on the extrusion temperature, the mixing action of theextruder and the residence time in the melt.

[0024] In a preferred embodiment of the invention, from 0.1 to 20% byweight of the polymers of the base layer B are identical with those ofthe outer layer A. These are either directly admixed with the base layerB during extrusion or are in any case present in the film due toaddition of regrind. The proportion of these copolyesters in the baselayer is selected in such a way that the base layer has crystallinecharacter.

[0025] In another embodiment, the film encompasses, on the side facingaway from the outer layer A, another outer layer of polyethyleneterephthalate, and this layer may, like the outer layer to bemetallized, comprise pigments.

[0026] The film of the invention has a high oxygen barrier and,surprisingly, the desired good adhesion between its individual layers.

[0027] If the polymers/copolymers used for the outer layer A containless than 90% of ethylene 2,6-naphthalate units and more than 10% byweight of ethylene terephthalate units, the thickness of the outer layerA being less than 0.7 μm, although the film then has less permeabilityto oxygen than a standard polyester film (composed of 100% by weight ofpolyethylene terephthalate), its permeability is too high for thepurposes of the present invention.

[0028] If the polymers/copolymers used for the metallized orceramic-coated outer layer A contain more than 98% by weight of ethylene2,6-naphthalate units (for example are pure polyethylene2,6-naphthalate), the adhesion between the outer layer A and the baselayer B becomes inadequate. When subjected to mechanical stress the filmtends toward delamination, which is undesirable and makes the filmunusable.

[0029] A difference from the prior art is that in the film of theinvention moreover the glass transition temperature T_(g) of thecopolymer or of the copolymers of the outer layer A is higher than theglass transition temperature T_(g) of the polymers for the base layer B.The glass transition temperature T_(g) of the copolymers used for theouter layer A is preferably in the range from 90 to 120° C. In DSC(differential scanning calorimetry) determination of the glasstransition temperatures, the transitions of the layers cannot bedifferentiated.

[0030] Glass transitions which are determined on biaxially oriented,heat-set films in the first heating procedure (termed T_(g)1 below) are,due to crystallinity and also to molecular stresses in the amorphousfraction of the specimens, relatively small in size, distributed over awide temperature range, and shifted to higher temperatures. Because oforientation effects in particular, they are not suitable forcharacterizing a polymer. The resolution of DSC analyzers is ofteninsufficient to detect the glass transitions in the first heatingprocedure (T_(g)1) of the individual layers of the film of theinvention, the transitions being “blurred” and small, due to orientationand crystallinity.

[0031] If the specimens are melted and then rapidly cooled again tobelow their glass transition temperature T_(g) (quenched), theorientation effects are eliminated. On renewed heating, glasstransitions (designated T_(g)2 here) are then measured which have agreater intensity and are characteristic of the respective polymers.However, even here it is not possible to differentiate the glasstransitions of the individual layers, since the layers mix on meltingand the polyesters present therein enter into transesterificationreactions with one another. It is fully sufficient, however, to comparethe T_(g)2 of the entire coextruded films with the T_(g)2 of the polymerused for the base layer B. In known films the T_(g)2 value of the baselayer is higher than that of the coextruded film, whereas the T_(g)2value of the outer layer is lower than that of the base layer and alsothan that of the coextruded film. Exactly the opposite relationshipsapply for the film of the invention. Here, the T_(g)2 value of thecoextruded film is higher than that of the base layer B but below theT_(g)2 value of the polymer for the outer layer A.

[0032] The high oxygen barrier demanded is not achieved if the metalliclayer or the ceramic layer is applied to that side of the base layerfacing away from the outer layer A (and not to the outer layer Aitself). This is true even if in other respects the makeup of base layerand of outer layer A corresponds to the film of the invention.

[0033] The base layer B and the outer layer(s) may also comprisecustomary additives, such as stabilizers and antiblocking agents. Theseare expediently added to the polymer or to the polymer mixture evenbefore melting takes place. Examples of stabilizers are phosphoruscompounds, such as phosphoric acid and phosphoric esters. Typicalantiblocking agents (also termed pigments in this context) are inorganicand/or organic particles, for example calcium carbonate, amorphoussilica, talc, magnesium carbonate, barium carbonate, calcium sulfate,barium sulfate, lithium phosphate, calcium phosphate, magnesiumphosphate, aluminum oxide, LiF, the calcium, barium, zinc and manganesesalts of the dicarboxylic acids used, carbon black, titanium dioxide,kaolin, crosslinked polystyrene particles and crosslinked acrylateparticles.

[0034] The additives selected may also be mixtures of two or moredifferent antiblocking agents or mixtures of antiblocking agents of thesame makeup but of different particle size. The particles may be addedto the individual layers in the customary concentrations, e.g. asglycolic dispersion during polycondensation or via masterbatches duringextrusion. Pigment concentrations of from 0.0001 to 5% by weight haveproven advantageous. A detailed description of the antiblocking agentsis found, for example, in EP-A-0 602 964.

[0035] The film may be coated and/or corona- or flame-pretreated toestablish other desired properties. Typical coatings are layers whichpromote adhesion, are antistatic, improve slip, or have release action.These additional layers may be applied to the film by in-line coatingusing aqueous dispersions, before transverse orientation.

[0036] The polyester film of the invention preferably also comprises asecond outer layer C. The structure, thickness, and makeup of the secondouter layer C may be selected without reference to the outer layer Aalready present, and the second outer layer C may also comprise theabovementioned polymers or polymer mixtures, but these do not have to beidentical with the chemical makeup of outer layer A. The second outerlayer C may also comprise other commonly used outer layer polymers.

[0037] Between the base layer B and the outer layer A, there may also bean intermediate layer Z. The thickness of the intermediate layer isgenerally above 0.1 μm and is preferably in the range from 0.2 to 20 μm,particularly from 0.3 to 10 μm.

[0038] The thickness of the outer layer C is generally above 0.1 μm,preferably in the range from 0.2 to 5 μm, particularly from 0.2 to 4 μm,and the thicknesses of the outer layers may be identical or different.

[0039] The total thickness of the polyester film of the invention mayvary within wide limts and depends on the application envisaged. It isfrom 6 to 100 μm, preferably from 8 to 50 μm, and particularlypreferably from 10 to 30 μm, the proportion of the total thickness madeup by the base layer preferably being from about 40 to 95%.

[0040] The metallic layer is preferably composed of aluminum. However,other materials which can be applied in the form of a thin coherentlayer are also suitable, particularly silicon, for example, which unlikealuminum gives a transparent barrier layer. The ceramic layer ispreferably composed of oxides of elements of the 2nd, 3rd or 4th maingroup of the Periodic Table, in particular of oxides of magnesium, ofaluminum, or of silicon. The metallic or ceramic materials used aregenerally those which can be applied at subatmospheric pressure or invacuo. The thickness of the layer applied is generally from 10 to 100nm.

[0041] The present invention also provides a process for producing thisfilm. It encompasses

[0042] a) producing a film having two or more layers from a base layer Band outer layer(s) A and, where appropriate, C, by coextrusion;

[0043] b) biaxially stretching the film, and

[0044] c) heat-setting the stretched film.

[0045] To produce the outer layer A, it is expedient to feed granules ofpolyethylene terephthalate and polyethylene 2,6-naphthalate directly tothe extruder in the desired mixing ratio. At about 300° C., the twomaterials can be melted and can be extruded. Under these conditions,transesterification reactions can occur in the extruder and during thesecopolymers are formed from the respective homopolymers.

[0046] The polymers for the base layer B are expediently fed in viaanother extruder. Any foreign bodies or contamination which may bepresent can be filtered off from the polymer melt before extrusion. Themelts are then extruded through a coextrusion die to give flat meltfilms and are layered one upon the other. The composite film is thendrawn off and solidified with the aid of a chill roll and other rolls ifdesired.

[0047] The biaxial stretching is generally carried out sequentially,stretching first longitudinally (i.e. in the machine direction) and thentransversely (i.e. perpendicularly to the machine direction). This leadsto orientation of the molecular chains within the polyester. Thelongitudinal stretching can be carried out with the aid of two rollersrotating at different rates corresponding to the desired stretchingratio. For the transverse stretching, use is generally made of anappropriate tenter frame.

[0048] The temperature at which the orientation procedure is carried outcan vary over a relatively wide range and depends on the propertiesdesired in the film. In general, the longitudinal stretching is carriedout at from 80 to 130° C., and the transverse stretching at from 90 to150° C. The longitudinal stretching ratio is generally in the range from2.5:1 to 6:1, preferably from 3:1 to 5.5:1. The transverse stretchingratio is generally in the range from 3.0:1 to 5.0:1, preferably from3.5:1 to 4.5:1.

[0049] During the subsequent heat-setting, the film is held for from 0.1to 10 s at a temperature of from 150 to 250° C. The film is then woundup in a conventional manner.

[0050] Prior to the transverse stretching, one or both surfaces of thefilm may be in-line coated by known processes. The in-line coating may,for example, serve to improve adhesion of the metallic layer to theouter layer A or of any printing ink which might be applied to the film,or else to improve antistatic performance or processing performance.

[0051] One or both side(s) of the biaxially oriented and heat-setpolyester film may be corona- or flame-treated prior to application ofthe metallic or ceramic layer. The intensity of treatment selected issuch that the resultant surface tension of the film is generally above45 mN/m.

[0052] A great advantage of this process is that it is possible to feedthe extruder with granules, which do not block the machine.

[0053] A further advantage is that the production costs of the film ofthe invention are only insignificantly greater than those of a film madefrom standard polyester raw materials. The other properties of the filmof the invention which are relevant to its processing and its use remainessentially unchanged or have even been improved. It has also beenensured that cut material arising directly in the plant during filmproduction can be used again in the form of regrind for film productionin amounts of up to 60% by weight, preferably from 10 to 50% by weight,based in each case on the total weight of the film, without anysignificant resultant adverse effect on the physical properties of thefilm thus produced with regrind.

[0054] The film has excellent suitability for packaging food or otherconsumable items. The film of the invention has excellent barrierproperties, in particular with respect to oxygen. It has been assuredthat the individual layers of the laminate remain adhering to oneanother when the film is processed, e.g. to give film laminates, and donot delaminate.

[0055] It is also possible to improve the gloss and the haze of thefilm, compared with prior art films. It has been ensured that regrindcan be reintroduced to the extrusion process during production of thefilm in amounts of up to 60% by weight, based on the total weight of thefilm, without any significant resultant adverse effect on the physicalproperties of the film.

[0056] The excellent handling properties of the film and its very goodprocessing properties make it particularly suitable for processing onhigh-speed machinery.

[0057] The metallic layer or the ceramic layer is usefully applied inwell known industrial systems. Metallic layers made from aluminum areusually produced by metalizing, while ceramic layers may also beproduced using electron beam processes or by sputtering. The processparameters for the system during application of the metallic or ceramiclayer to the films correspond to standard conditions. The metalizationof the films is preferably carried out so as to give an optical densityin the usual range from about 2.2 to 2.8 for the metalized films. Theapplication of the ceramic layer to the film is carried out so as togive an oxide layer thickness preferably in the range from 30 to 100 nm.The web speed of the film to be coated is from 5 to 10 m/s for allsettings of variables. A laboratory metalization system was not used formetalizing, since experience has shown that the barrier values are thengenerally significantly better and cannot be used for comparativepurposes.

[0058] The table below (Table 1) gives the most important filmproperties of the invention again at a glance for quick reference. TABLE1 Range according to Particularly the invention Preferred preferred UnitTest method Outer layer A Ethylene 2,6-naphthalate units 90 to 98 91 to97 92 to 96 % by weight Ethylene terephthalate units <10 <9 <8 % byweight Thickness 0.7 μm up to 25% 0.8 μm up to 0.9 μm up to μm/% by oftotal thickness 22% of total 20% of total weight thickness thicknessFilm properties Oxygen permeation <0.3 <0.25 <0.20 cm³/(m² · bar · d)DIN 53 380, Part 3 Adhesion between the layers >0.5 >0.7 >1.0 N/25 mminternal

Test Methods

[0059] The following methods were utilized to characterize the rawmaterials and the films:

Oxygen Permeability

[0060] The oxygen barrier test took place using a Mocon Modern Controls(USA) OX-TRAN 2/20, as in DIN 53 380, Part 3.

Measurement of Optical Density

[0061] Optical density was measured using the TD-904 densitometer fromMacbeth (Division of Kollmorgen Instruments Corp.). The optical densityis defined as OD=−lg l/l₀, where l is the intensity of the incidentlight, l₀ is the intensity of the emitted light, and l/l₀ is thetransmittance.

SV (Standard Viscosity)

[0062] The standard viscosity SV (DCA) is measured by analogy with DIN53726, at 25° C. in dichloroacetic acid. The intrinsic viscosity (IV) iscalculated from the standard viscosity as follows

IV=[η]=6.907·10⁻⁴ SV (DCA)+0.063096 [dl/g].

Coefficient of Friction

[0063] The coefficient of friction was determined to DIN 53 375. Thecoefficient of sliding friction was measured 14 days after production.

Surface Tension

[0064] Surface tension was determined using what is known as the inkmethod (DIN 53 364).

Haze

[0065] The haze of the film was measured to ASTM-D1003-52. The Hölz hazemeasurement was determined by analogy with ASTM-D1003-52, but, in orderto utilize the ideal measurement range, measurements were taken on fourpieces of film laid one on top of the other, and a 1° slit diaphragm wasused instead of a 40° pinhole.

Gloss

[0066] Gloss was determined to DIN 67 530. The reflectance was measured,this being a characteristic optical value for a film surface. Based onthe standards ASTM-D523-78 and ISO 2813, the angle of incidence was setat 20° or 60°. A beam of light at the set angle of incidence hits theflat test surface and is reflected and/or scattered thereby. Aproportional electrical variable is displayed representing light rayshitting the photoelectric detector. The value measured is dimensionlessand must be stated together with the angle of incidence.

Glass Transition Temperatures

[0067] The glass transition temperatures T_(g)1 and T_(g)2 weredetermined with the aid of DSC (differential scanning calorimetry) onfilm specimens. A DuPont DSC 1090 was used. The heating rate was 20K/min, and the specimen weight was about 12 mg. The glass transitionT_(g)1 was determined in the first heating procedure. Many of thespecimens showed an enthalpy relaxation (a peak) at the beginning of thestep-like glass transition. The temperature taken as T_(g)1 was that atwhich the step-like change in heat capacity—ignoring the enthalpyrelaxation peak—achieved half of its height in the first heatingprocedure. In all cases, there was only a single glass transition stagein the thermogram in the first heating procedure. It is possible thatthe enthalpy relaxation peaks obscured the fine structure of thetransition, or that the resolution of the device was not adequate toseparate the small, “blurred” transitions of oriented, crystallinespecimens. In order to eliminate their heat history the specimens wereheld at 300° C. for 5 minutes after the heating procedure, and thenquenched with liquid nitrogen. The temperature for the glass transitionT_(g)2 was taken as the temperature at which the transition reached halfof its height in the thermogram for the second heating procedure.

Adhesion Between the Layers

[0068] Prior to adhesive bonding, the specimen of film (300 mm long×180mm wide) of the present invention is placed on a smooth piece of card(200 mm long×180 mm wide; about 400 g/m², bleached, outer laps coated).The overlapping margins of the film are folded back onto the reverseside and secured with adhesive tape.

[0069] For adhesive bonding of the film according to the presentinvention, use is made of a standard polyester film of 12 μm thickness(e.g. Melinex 800), and a doctor device and doctor bar No. 3 fromErichsen, applying about 1.5 ml of adhesive (Novacote NC 275+CA 12;mixing ratio: 4/1+7 parts of ethyl acetate) to the outer layer A of thefilm of the present invention. After aerating to remove the solvent, thestandard polyester film is laminated to outer layer A of the film of thepresent invention using a metal roller (width 200 mm, diameter 90 mm,weight 10 kg, to DIN EN 20 535). The lamination parameters are: Amountof adhesive: 5 +/− 1 g/m² Aeration after application of adhesive: 4 min+/−15 s Doctor thickness (Erichsen): 3 Doctor speed level: about 133mm/s Bond curing time: 2 h at 70° C. in a circulating air drying cabinet

[0070] A 25±1 mm strip cutter is used to take specimens about 100 mm inlength. About 50 mm of composite is needed here, and 50 mm of unbondedseparate laps for securing/clamping the test specimen. The testspecimens are secured to a sheet metal support by means of double-sidedadhesive tape, by way of the entire surface of the reverse side of thefilm of the present invention (base layer B or outer layer C). The sheetwith the composite adhesively bonded thereto is clamped into the lowerclamping jaw of the tensile test machine. The clamp separation is 100mm. The unlaminated end of the standard polyester film is clamped intothe upper clamping jaw of the tensile test machine (e.g. Instron, Zwick)so that the resultant peel angle is 180°. The average peel force in N/25mm is given, rounded to one decimal place. Specimen width: 25 mmPretensioning force: 0.1 N Test length: 25 mm Separation rate untilpretensioning force applied: 25 mm/min Start position: 5 mm Testdisplacement: 40 mm Sensitivity: 0.01 N Separation rate: 100 mm/min

[0071] The peel force test result is equivalent to the minimum adhesionbetween the layers, since the adhesion between the adhesive and thestandard film is markedly greater. A UV lamp, for example, can be usedto demonstrate the release of the outer layer A from the base layer B ofthe film of the present invention. The UV light has a blueish appearanceif copolymer of PEN and PET is present on the adhesive and this layer isirradiated using a UV lamp.

EXAMPLE

[0072] The following examples illustrate the invention. Information oneach of the products used (trademark and manufacturer) is given onlyonce, and this is then applicable to the examples which follow.

Example 1

[0073] Chips of polyethylene terephthalate and polyethylene2,6-naphthalate in a mixing ratio of 3:97 were dried at a temperature of160° C. to residual moisture below 100 ppm, and fed directly to theextruder for the outer layer A, where the two materials were extruded ata temperature of about 300° C. The melt was filtered and extrudedthrough a coextrusion die to give a flat film and, as outer layer A,superposed on the base layer B. The coextruded film was discharged byway of the die lip and solidified on a chill roll. The residence time ofthe two polymers in the extrusion process was about 5 min. Under theconditions given, a copolymer was produced in the extrusion process.

[0074] Chips of polyethylene terephthalate were dried at a temperatureof 160° C. to residual moisture below 100 ppm and fed to the extruderfor the base layer B. In addition, chips of polyethylene terephthalateand antiblocking agents were likewise dried at 160° C. to residualmoisture of 100 ppm and fed to the extruder for the outer layer C. Theconditions in the extruder for the outer layer C were the same as thosefor coextruder A.

[0075] Coextrusion followed by stepwise longitudinal and transversestreching was used to produce a transparent three-layer ABC film with atotal thickness of 12 μm. The thickness of the outer layer A was 1.1 μmand that of the outer layer C was 1.0 μm. One side of the film, outerlayer A, was then metalized in vacuo with aluminum in an industrialmetalizer. The coating speed was 5 m/s.

[0076] Outer layer A:

[0077] 97% by weight of polyethylene 2,6-naphthalate (®Polyclear P 100prepolymer from KOSA, Offenbach) with an SV of 600, and

[0078] 3% by weight of polyethylene terephthalate with SV of 800.

[0079] Base layer B:

[0080] 100% by weight of polyethylene terephthalate (4020 from KOSA,Offenbach) with SV of 800.

[0081] Outer layer C:

[0082] 80% by weight of polyethylene terephthalate with SV of 800, and

[0083] 20% by weight of masterbatch made from 99.0% by weight ofpolyethylene terephthalate and 1.0% by weight of silica particles, 50%of which had an average particle size of 2.5 μm, and 50% of which had anaverage particle size of 0.4 μm.

[0084] The individual steps of the process were: Extrusion Temperatures:Outer layer A: 300° C. Base layer B: 300° C. Outer layer C: 300° C.Take-off roll temperature 30° C. Longitudinal stretching temperature:122° C. Longitudinal stretching ratio: 4.5:1 Transverse stretchingtemperature: 125° C. Transverse stretching ratio: 4.0:1 Settingtemperature: 230° C.

[0085] The film had the oxygen barrier required and the adhesionrequired.

Example 2

[0086] Coextrusion was used as in Example 1 to produce a three-layer ABCfilm with a total thickness of 12 μm. The thickness of the outer layer Awas 1.3 μm and the thickness of the outer layer C was 1.0 μm. The outerlayer A of the film was metalized with aluminum in vacuo in aconventional industrial metalizer. The coating speed was 5 m/s.

[0087] Outer layer A:

[0088] 95% by weight of polyethylene 2,6-naphthalate (®Polyclear P 100prepolymer from KOSA, Offenbach) with an SV of 600, and

[0089] 5% by weight of polyethylene terephthalate with SV of 800.

[0090] Base layer B:

[0091] 100% by weight of polyethylene terephthalate (4020 from KOSA,Offenbach) with SV of 800.

[0092] Outer layer C:

[0093] 80% by weight of polyethylene terephthalate with an SV of 800,and

[0094] 20% by weight of masterbatch made from 99.0% by weight ofpolyethylene terephthalate and 1.0% by weight of silica particles, 50%of which had an average particle size of 2.5 μm, and 50% of which had anaverage particle size of 0.4 μm.

[0095] The process conditions for all of the layers were as in Example1.

Comparative Example 1c

[0096] A film was produced by analogy with Example 8 of EP-A-0 878 298.The film metalized on the outer layer A had the oxygen barrier required,but the adhesion between layers A and B was extremely low.

Comparative Example 2c

[0097] A film was produced by analogy with Example 1 of U.S. Pat. No.5,795,528, except that unlike in the example from U.S. Pat. No.5,795,528 there were only 2 layers selected from PEN and PET. The filmmetalized on the PEN surface had the oxygen barrier required, but theadhesion between layers A and B was extremely low. Table 3 gives theproperties of the films produced in Examples 1 and 2 and in theComparative Examples 1 c and 2c. TABLE 2 Ethylene Ethylene2,6-naphthalate units terephthalate units Example in outer layer A inouter layer A No. (in % by weight) (in % by weight) 1 97 3 2 95 5 1c 1000 2c 100 0

[0098] TABLE 3 Layer Adhesion Gloss Film thicknesses Oxygen Opticalbetween (20° measurement Example thickness A/B/C Film permeation densityof layers angle) No. (μm) (μm) structure (cm³/m² bar d) metal layer N/25mm Side A Side C Haze¹⁾ 1  12 1.1/9.9/1.0 ABC 0.08 2.60 0.6 205 175 1.82  12 1.3/9.7/1.0 ABC 0.068 2.60 1.4 199 180 1.6 1c 12 3.0/7.5/1.5 ABC0.07 2.60 0.1 203 175 1.8 2c 12 6.0/6.0 AB 0.08 2.60 0.1 200 195 2.0

What is claimed is:
 1. A biaxially oriented polyester film with a base layer B, which comprises at least 80% by weight of thermoplastic polyester, and with at least one outer layer A and with a metallic or ceramic layer arranged on the outer layer A, wherein the outer layer A is composed of a copolymer or of a mixture of polymers/copolymers, which contains from 90 to 98% by weight of ethylene 2,6-naphthalate units and up to 10% by weight of ethylene terephthalate units, and/or units derived from cycloaliphatic or aromatic diols and/or dicarboxylic acids; the thickness of the outer layer A is more than 0.7 μm and this makes up less than 25% by weight relative to the entire film, and the T_(g)2 value of the polyester film is above the T_(g)2 value of the base layer but below the T_(g)2 value of the outer layer.
 2. The film as claimed in claim 1, wherein the copolymer or the mixture of polymers of outer layer A contains from 91 to 97% by weight of ethylene 2,6-naphthalate units.
 3. The film as claimed in claim 1, wherein the the thickness of the outer layer A is more than 0.8 μm, this making up less than 22% by weight relative to the entire film.
 4. The film as claimed in claim 1, wherein the oxygen permeation of the film is below 0.3 cm³/(m²·bar·d).
 5. The film as claimed in claim 1, wherein the adhesion between the individual layers is greater than or equal to 0.5 N/25 mm.
 6. The film as claimed in claim 1, which additionally comprises an intermediate layer Z which has a thickness above 0.1 μm.
 7. The film as claimed in claim 1, which has three layers and is composed of the base layer B, the outer layer A, and the outer layer C.
 8. The film as claimed in claim 1, which has four layers and comprises the outer layer C, arranged thereupon the base layer B, arranged thereupon the intermediate layer Z, and arranged thereupon the outer layer A.
 9. The film as claimed in one or more of claims 1 to 8, wherein at least one of the outer layers has been pigmented.
 10. The film as claimed in claim 1, wherein at least one side of the film has been corona-treated.
 11. The film as claimed in claim 1, wherein at least one side of the film has been in-line coated.
 12. A process for producing the film as claimed in one or more of claims 1 to 11, encompassing the steps producing a film from base and outer layer(s) by coextrusion, biaxially stretching the film, heat-setting the stretched film, and applying the metal layer or the ceramic layer to the heat-set film, which comprises carrying out the biaxial stretching by a longitudinal stretching of the film at a temperature in the range from 80 to 130° C. and by a transverse stretching in the range from 90 to 150° C. and using a longitudinal stretching ratio in the range from 2.5:1 to 6:1, and using a transverse stretching ratio in the range from 3.0:1 to 5.0:1.
 13. The process as claimed in claim 12, wherein, for heat-setting, the stretched film is held for a period of from about 0.1 to 10 s at a temperature of from 150 to 250° C.
 14. The process as claimed in claim 12, wherein cut material arising during film production is reused as regrind in the film production in amounts of up to 60% by weight based in each case on the total weight of the film. 